Constraints on Earth-mass primordial black holes from OGLE 5-year microlensing events

Abstract

We constrain the abundance of primordial black holes (PBH) using 2622 microlensing events obtained from 5-years observations of stars in the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE). The majority of microlensing events display a single or at least continuous population that has a peak around the light curve timescale tE 20 days and a wide distribution over the range tE [1, 300] days, while the data also indicates a second population of 6 ultrashorttimescale events in tE [0.1, 0.3] days, which are advocated to be due to free-floating planets. We confirm that the main population of OGLE events can be well modeled by microlensing due to brown dwarfs, main sequence stars and stellar remnants (white dwarfs and neutron stars) in the standard Galactic bulge and disk models for their spatial and velocity distributions. Using the dark matter (DM) model for the Milky Way (MW) halo relative to the Galactic bulge/disk models, we obtain the tightest upper bound on the PBH abundance in the mass range MPBH [10 −6 , 10 −3 ]M (Earth-Jupiter mass range), if we employ "null hypothesis" that the OGLE data does not contain any PBH microlensing event. More interestingly, we also show that Earth-mass PBHs can well reproduce the 6 ultrashort-timescale events, without the need of free-floating planets, if the mass fraction of PBH to DM is at a per cent level, which is consistent with other constraints such as the microlensing search for Andromeda galaxy (M31) and the longer timescale OGLE events. Our result gives a hint of PBH existence, and can be confirmed or falsified by microlensing search for stars in M31, because M31 is towards the MW halo direction and should therefore contain a much less number of free-floating planets, even if exist, than the direction to the MW center. * niikura@hep.phys.s.u-tokyo.ac.jp † masahiro.takada@ipmu.jp producing PBHs with a target mass scale and a target abundance. Furthermore, there is a renewed interest in PBH scenario because of recent claims [12][13][14][15][16] [also see 17] that PBHs can be progenitors of binary black holes whose gravitational wave events have been detected by the LIGO/Virgo experiments [18,19].There are various attempts at constraining PBHs over almost twenty orders of magnitudes in their mass scales; gamma-ray background from PBH evaporation [20], femtolensing of gamma-ray bursts [21] (although Ref. [22] recently pointed out that a finite-source size effect of the gamma-ray burst progenitor significantly relaxes or even removes the constraint), supernovae of white dwarfs triggered by PBH [23], PBH capture by neutron stars [24,25], microlensing constraints [26][27][28][29], caustics-network lensing in the galaxy cluster region [30][31][32], X-ray background from gas accretion on PBH [33], and the effect of PBH gas accretion on the cosmic microwave background optical depth [34] (see Refs. [35,36] for the revisited calculations), the effect of PBH on pulsar timing array observation [37] and the effect on type-Ia supernova observation [38]. Except fo...

Gravitational microlensing by dark matter in extended structures

“…Using Ref. [30]'s binning of events in t E , for every bin i we define N DM i as the number of dark matter-induced events obtained from Eq. 12) and obtain the 90% c.l.…”

Constant roll and primordial black holes

“…While the CR potential (5) allows the exact solution (6), the background evolution of the inflationary system for various initial conditions has to be checked numerically. For convenience in the numerical integration, we use dimensionless variables ϕ ≡ φ/M Pl and h ≡ H/M , and the e-folding number N ≡ ln a ai .…”

Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates

“…Both the Poisson noise and accretion methods have promise for future -and current -21 cm observations [53][54][55]. Furthermore, microlensing searches [56] have constrained PBH masses in the planetary-to-stellar-mass window by monitoring stars in the Large Magellanic Cloud (MACHO [57,58], EROS-2 [59]) and the Galactic Bulge [60]. Lower masses have been constrained by monitoring stars in the Kepler field [61] and stars in M31 observed by Subaru/HSC [62].…”